1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to a technique of correcting misregistrations between the respective colors in an electrophotographic color image forming apparatus.
2. Description of the Related Art
Conventionally, for example, a multicolor image forming apparatus has been proposed, which includes a plurality of image forming units and can form a color image in the following procedure. Each image forming unit applies a laser beam onto a drum-like electrophotographic photoreceptor as an image carrier, that is, a photosensitive drum. Each unit forms an electrostatic latent image on the photosensitive drum by an electrophotographic process. A developing device develops this electrostatic latent image and turns it into a visible image (toner image). Transfer units multi-layer-transfer the toner images, formed on the photosensitive drums by the respective image forming units, onto a transfer material conveyed by a belt-like transfer material conveyor (transfer convey belt), or perform multiple transfer of the respective images on a belt-like intermediate transfer member (intermediate transfer belt). Thereafter, the images are collectively transferred onto a transfer material.
In this type of image forming apparatus, misalignments occur between the respective color images formed on the respective photosensitive drums due to various factors. Registration for the cancellation of such misalignments may sometimes become incomplete on a transfer material on which images finally undergo multiple transfers. The various factors in this case include, for example, mechanical mounting errors between the respective photosensitive drums, optical path length errors between the respective laser beams, and optical path changes.
FIGS. 1, 2A, and 2B show a case of coping with a situation in which registration is incomplete. Conventional photosensitive drums 11, that is, 11a, 11b, 11c, and 11d, are used to form registration correction patterns 70 as misalignment detection patterns on an endless belt such as an intermediate transfer belt 31 (or transfer convey belt) as the second image carrier. Photosensors 60 and 61 as pattern detection units placed adjacent to the photosensitive drum 11a of the image forming unit on the most downstream side read the patterns and detect misregistrations the photosensitive drums 11 corresponding to the respective colors formed by the respective image forming units. This apparatus electrically corrects an image signal to be printed by using detected misalignment values.
Various patterns have been proposed as registration correction pattern images. For example, Japanese Patent Laid-Open No. 2000-98810 has proposed a pattern including the first line segment placed at a predetermined angle relative to a process direction as the moving direction of an endless belt and the second line segment placed symmetrically with the first line segment about a virtual line perpendicular to the process direction.
A pattern detection unit is constituted by a light-emitting element, such as an LED or phototransistor, and a light-receiving element, that is, a photosensor as a pattern reading unit. The pattern detection unit reads such a registration correction pattern image. Two such photosensors are arranged at a predetermined distance from each other in a direction perpendicular to the process direction. A registration correction pattern image is formed so as to pass over the photosensors.
FIG. 3 shows how the photosensors 60 and 61 detect the registration correction pattern 70 on the intermediate transfer belt 31. Note that the intermediate transfer belt 31 is made of a material whose reflectance of light (for example, infrared light) emitted by light-emitting elements (LEDs) in the photosensors 60 and 61 is higher than that of the registration correction pattern 70. Differences in reflectance of emitted light allow pattern detection.
FIG. 4 shows how a light-receiving element (phototransistor) PT receives reflected light emitted by the LED and reflected by the registration correction pattern 70 for image misalignment or the intermediate transfer belt 31. FIG. 5 shows a light-receiving circuit which converts the reflected light received by the phototransistor PT into electrical signal.
First of all, when the photosensors 60 and 61 detect reflected light from the intermediate transfer belt 31, since the reflected light amount is large, a large photocurrent flows in the phototransistor PT. A resistor R1 converts current into voltage. Resistors R2 to R4 and an operational amplifier OP1 amplify the voltage. When the photosensors 60 and 61 detect reflected light from the registration correction pattern 70 formed on the intermediate transfer belt 31, since the reflected light amount is small, a photocurrent smaller than that flowing upon detection of reflected light from the intermediate transfer belt 31 flows in the phototransistor PT. As in the above case, the resistor R1 converts the photocurrent into voltage, and the resistors R2 to R4 and the operational amplifier OP1 amplify the voltage.
The waveform pattern indicated by A in FIG. 4 shows how the light-receiving circuit detects reflected light from the intermediate transfer belt 31, the registration correction pattern 70, and the intermediate transfer belt 31 in the order named. As shown in FIG. 5, fixed resistors R5 and R6 set a threshold level at almost the midpoint between the detection level at the intermediate transfer belt 31 and the detection level at the registration correction pattern 70. A comparator OP2 then compares the value obtained by converting the current detected by the phototransistor PT into a voltage with the threshold level. This operation can generate a registration correction pattern detection output like the waveform pattern indicated by B in FIG. 4. The control unit of this system reads this pattern detection output which is sequentially transmitted, and detects a misregistration amount from pattern intervals and the like, thereby electrically correcting an image signal to be printed.
The registration correction described above is based on the premise that the registration correction pattern has a proper density. However, the density of the registration correction pattern varies due to environmental changes such as variations in temperature and humidity, deteriorations in toner materials over time, and the like. When, in particular, the density of the registration correction pattern decreases due to variations in temperature and humidity or the like or deteriorations in toner materials over time or the like, the registration correction accuracy also deteriorates.
If the density of the registration correction pattern for each color is not uniform, the rising or falling speed of an analog signal corresponding to the registration correction pattern changes. When the rising or falling speed of the analog signal changes, the timing of a leading or trailing edge of a pulse of a digital signal generated from the analog signal changes. More precisely, the change amount of the leading or trailing edge of a pulse of an analog signal varies for each color. For this reason, a color misalignment amount detected from pulses of digital signals includes the difference in change amount between the timings of edges. This leads to a deterioration in detection accuracy associated with the relative positional relationship between registration correction patterns.
The waveform pattern indicated by A in FIG. 4 has a bilaterally symmetrical pattern. However, an actual waveform pattern is not bilaterally symmetrical in a strict sense. This is because of the influences of the optical characteristics of each photosensor based on the installation positions of the LED and PT of the photosensor. An analog signal after a variation in the density of a registration correction pattern does not have a bilaterally symmetrical pattern, either. If, therefore, the density of a registration correction pattern excessively decreases, the change amounts of the gradients of the leading and trailing edges of an analog signal increase. This leads to deterioration in detection accuracy of the relative positional relationship between registration correction patterns.
In order to solve this problem, decreases in pattern image density are made to fall within the specifications of various types of image forming apparatuses by controlling changes (high voltage settings) of laser light amounts, charging biases to be set in chargers, and developing biases to be set in developing devices, and various kinds of process conditions for toner replenishment control in accordance with durability. Note however that as image density decreases, the density of a corresponding registration correction pattern decreases. That is, even if the image density can be made to fall within specifications, color misalignment may fall outside the specifications.
In order to prevent deterioration in registration correction accuracy, the laser light amount may be increased. If, however, the laser light amount is always increased in operation including printing operation, the charge of the latent image formed on the photosensitive drum may not be sufficiently removed. This may adversely influence the next printed image or cause early deterioration in the photosensitive drum.